1. Field of the Invention
The present invention generally relates to an electrode for acquiring a biomedical signal (bioelectrode) and, more particularly, to a waterproof bioelectrode.
2. Description of the Related Art
Conventionally, a biomedical signal typically represented by an electrocardiogram is widely used as information useful for diagnosis. A biomedical signal may be acquired not only in at rest but also during exercise or daily life depending on its purpose. For example, acquisition of an electrocardiogram by a Holter electrocardiograph is a typical way of acquiring a biomedical signal during daily life for a long period of time continuously.
To acquire a biomedical signal, a bioelectrode to be attached on the body surface is necessary. The bioelectrode must acquire a stable biomedical signal with small noise over an acquisition period. For this purpose, a wide variety of bioelectrodes are available in accordance with the type of the biomedical signal to be acquired, the acquisition environment, the acquisition period, and the like.
In acquisition of the biomedical signal by means of the Holter electrocardiograph described above, the bioelectrode must follow the body motion accompanying the daily life of the object and be kept firmly attached on the body surface without peeling by perspiration over a long period of time.
A conventionally used bioelectrode is generally non waterproof and the position where the electrode is attached influences the quality of the biomedical signal. Therefore, even a biomedical signal acquisition by means of the Holter electrocardiograph may last for two days, the object is often prohibited taking a bath or shower during measurement. If the object must necessarily take a bath or shower, the electrode is removed temporarily in advance and re-attached afterwards. To remove and re-attach the electrode, however, bothers the object. Further, in re-attaching, the attaching position may be shifted, or the electrode may not be correctly attached in tight contact with the body surface. This may influence the biomedical signal to be acquired to interfere with accurate diagnosis. Therefore, it is desirable that the electrode need not be removed even if the electrode portion may get wet while the object takes a bath or shower.
When taking a bath or shower, the burden to the heart increases, and accordingly an abnormality may occur in the electrocardiogram. If the electrocardiogram in such an abnormal state cannot be acquired, information that is critical in diagnosis may be overlooked. Hence, a waterproof electrode is desirable also from the viewpoint of enabling acquisition of biomedical signals in various states. Accordingly, waterproof bioelectrodes have been proposed (see Japanese Patent Laid Open Nos. 2004-121360 (D1) and No. 2004-097809 (D2)).
According to the bioelectrode described in D1, an electrocardiogram is acquired with one electrode pad. A plurality of electrodes are necessary to obtain a biomedical signal, and a short distance between electrodes is undesired to obtain a signal having a good quality. For this reason, a certain amount of distance must be ensured between two electrodes provided to the pad, leading to a large size pad. It is not easy to attach a large size sheet in tight contact with the body surface. In addition, because a large-size sheet cannot easily follow the body motion, a gap tends to be formed between the sheet and the body surface. Hence, it is hard to maintain waterproofness.
Further, the bioelectrode pad described in D1 has a signal processing circuit, which transmits a signal to a monitor device by radio communication and is directly connected to the electrode pad. Hence, the electrode has a large projection to make the object who wears it feel uncomfortable. Also, the signal processing circuit portion may be pulled by the clothes or underwear of the object. Moreover, since the signal processing circuit must transmit a biomedical signal by radio communication, it requires a power supply and accordingly power supply management.
In the bioelectrode described in D2, a cable is connected to the electrode using a connector. The connector is connected adjacent to the electrode, and a large projection is present in the vicinity of the electrode, in the same manner as in D1. Hence, the object who wears the bioelectrode feels uncomfortable, and when the projection abuts against the electrode, noise may be superposed on the biomedical signal, which are the same problems as those in D1.
In both patent references 1 and 2, a sheet made of a polyethylene foam or polyurethane foam is used as the material of the electrode pad. Although external water will not be permeated through the electrode pad, moisture from inside (the body surface of the object) generated by perspiration is not transpired either. Consequently, the attaching portion of the body surface becomes sweaty during a long period use and may cause an itch or the like.